This invention relates to the purification of solid materials, and, more particularly, to the removal of impurities from (Nb.sub.1-x Ta.sub.x).sub.2 O.sub.5.
High purity, nearly perfect single crystals are used in a wide variety of electronic, optical, electro-optical, and other applications because of particular properties found in such single crystals of selected compositions. In some cases, the single crystals are usually prepared by first providing a precursor material in a highly purified form, and then transforming the pure precursor material into a single crystal of the composition desired.
A technologically important example is found in the precursor material M.sub.2 O.sub.5, where M is (Nb.sub.1-x Ta.sub.x), with x varying from 0 to 1. When x is 0, the precursor material is Nb.sub.2 O.sub.5, which can be transformed to single crystals of KNbO.sub.3. When x is 1, the precursor material is Ta.sub.2 O.sub.5, which can be transformed to single crystals of KTaO.sub.3. When x is between 0 and 1, the precursor material is (Nb.sub.1-x Ta.sub.x).sub.2 O.sub.5, which can be transformed to single crystals of intermediate composition KNb.sub.1-x Ta.sub.x O.sub.3, also known as KTN. These single crystals are used in electro-optical applications, wherein the refractive index of the crystal can be altered by application of an electric field, and in some cases can be used to sense heat through the pyroelectric effect.
The performance of the single crystals prepared by this approach is heavily dependent upon the types and amounts of impurities in the precursor material. To cite two particular examples, metal-ion impurities having at least two stable valence states, such as elements in the 3d period of the periodic table, can play the role of electron donors or acceptors. Impurities with one stable valence state that is not matched in oxidation number to the host constituent for which they substitute cause charge compensation processes. Color centers can result from these processes, which adversely affect the optical transparency and polarization behavior of the crystal, and increase the electrical conductivity of the crystal so as to possibly permit the crystal to electrolyze.
An important feature of the preparation of single crystals of these types is therefore the reduction of the impurity contents, particularly for those impurity elements that interfere with the electro-optical properties of the crystals. A number of approaches have been tried for reducing the impurity levels, but none has proved sufficiently effective for general use in preparing such crystals. In the past, (Nb.sub.1-x Ta.sub.x).sub.2 O.sub.5 has been purified as the acid, the term used to describe the related hydrated oxide H(Nb.sub.1-x Ta.sub.x)O.sub.3.yH.sub.2 O. The acid is obtained from ore by alkaline fusion followed by boiling. The acid then precipitates out during hydrolysis. This highly dispersed material is purified of various impurities in a series of tedious procedures wherein one or a few impurities at a time are removed. For example, tin and antimony are removed by digesting the precipitated acid with ammonium sulfide solution. Tungsten is extracted by digestion with dilute ammonia or ammonium carbonate, or by digesting with warm sodium hydroxide solution and boiling with an excess of ammonium nitrate. Silica is removed with hydrofluoric and sulfuric acids. Titanium is removed by adding an excess of salicylic acid and boiling in a reflux condenser, followed by filtration and precipitation of the titanium with ammonia. Zirconium is separated by fusion with bisulfate, the melt being extracted with dilute sulfuric acid to dissolve the zirconium sulfate. After purification, ignition of the purified acid yields (Nb.sub.1-x Ta.sub.x).sub.2 O.sub.5.
In another prior approach, hydrolysis of a purified pentachloride (Nb.sub.1-x Ta.sub.x)Cl.sub.5 produces a purified acid. However, neither of these approaches, nor any other presently known, is effective in removing the harmful 3d impurities. Accordingly, there exists a need for an effective, economical process for removing impurities from single crystal precursor materials of the type (Nb.sub.1-x Ta.sub.x).sub.2 O.sub.5. The present invention fulfills this need, and further provides related advantages.